What is Fermentation ?

Fermentation is a process used by anaerobic organisms and certain cells of aerobic organisms, such as muscle cells deprived of oxygen, to obtain additional energy from the glycolytic pathway. In other words, if there is no oxygen to accept electrons at the end of the electron transport system, all of the NAD available will be bound in reduced form, and cells must turn to glycolysis to supply ATP. As you have learned, glycolysis requires NAD+, in order to oxidize (remove electrons from) G3P to form 3PG. Oxidizing G3P forms NADH + H+. NAD+ must then be regenerated to allow glycolysis to continue.

In one type of fermentation, NADH + H+ reduce pyruvate to form either lactic acid or ethanol, depending upon the organism, plus NAD+. This regenerates NAD+, allowing glycolysis to continue. Lactic acid is the end point in overused muscle tissue; ethanol (ethyl alcohol) during fermentation of beer or wine. The reactions are as follows:

Pyruvate +  NADH ⁺ H⁺→lactic acid + NAD⁺

or

Pyruvate + NADH + H⁺→ ethanol + CO₂ + NAD⁺